Sub-frames attached to the suspension of a vehicle generally include side members extending along the front-rear direction of the vehicle body and a cross member extending along the vehicle width direction. The cross member may be linked to the side members or a vehicle body frame including the axle at the ends of the cross member in the vehicle width direction.
The cross member is subjected to forces in front-rear, left-right, and up-down directions. Thus, the cross member needs to have sufficient rigidity to resist such forces and their moments, and the cross member needs to be lightweight.
A cross member having a hollow internal structure for providing sufficient rigidity at a lightweight is known (see Patent document 1, for example).